1. Field of the Invention
This invention relates to cold-rolled steel sheets for deep drawing having an improved bake hardenability and a method of manufacturing the same.
2. Description of the Prior Art
Lately, there has been strong demand for an increase in the strength of automotive outer steel sheets as a weight-saving measure in automotive vehicles in order to improve fuel consumption. On the other hand, such steel sheets are desired to have a low yield strength, a high elongation, a high r-value and the like from a viewpoint of press formability.
From the above conflicting requirements, therefore, a desired steel sheet is demanded to be soft and have a good workability in the press forming and exhibit a property of increasing the yield strength or a so-called bake hardenability in the subsequent paint baking.
As regards the cold-rolled steel sheet having the bake hardenability and the method of manufacturing the same, there are descriptions on Ti-containing steel in Japanese Patent laid open No. 53-114,717, Nb-containing steel in Japanese Patent laid open No. 57-70,258, and Ti and Nb-containing steel in Japanese Patent laid open No. 59-31,827. In any case, the bake hardenability is imparted without deterioration of other properties by controlling the amounts of Ti, Nb added or the cooling rate in the annealing to make the amount of solute carbon in steel proper.
However, if it is intended to leave the solute carbon by controlling the addition amounts of Ti, Nb, the properties of the steel sheet are considerably influenced by the delicate change of the addition amount. That is, when the addition amount of Ti, Nb is outside the predetermined range, the properties exerting on formabilities such as elongation, r-value and the like are degraded or the bake hardenability is not obtained satisfactorily. Therefore, the exact or precise control of the addition amount is considered to be significant in the production step.